Why did the salmon cross the road?

Every November and December Hood Canal's Skokomish River rises out of its banks, and many returning chum salmon are forced to swim across flooded roads to reach their spawning grounds.
It is a compelling scene that illustrates how salmon manage to overcome sometimes monumental obstacles to continue their cycle of life.

Crowd cheers salmon crossing road
It's common sight in some parts of Western Washington when there is flooding.

.